In a video special effects system it is usually desired as a special effect to modify some or all of the colors in the image. For example, an effect called "solarization" sometimes referred to as "false color", causes the darkest parts of the image to be displayed as a particular color, e.g. blue, and the brightest parts of the image to be displayed as a different color, e.g. orange. Various shades in between would be converted to the color range between blue and orange, in this example; or perhaps a third color would be substituted somewhere in the range.
Another example of an effect which modifies color is "posterization". In this effect, the original image, which may contain thousands of shades, is represented using a very few discrete colors. For example, an image of a face could be displayed using only black, orange, and white. The resulting image would be recognizable, but would have a high-contrast cut-out look.
Other examples of color modification would include negative color effects, brightness and contrast changes, or overall color tints, on either all or selected parts of the image.
In computer graphics systems using CRT monitors accepting the color components red, green and blue (hereafter R, G, and B) these components can be produced individually by reference to three Look-Up Tables (hereafter "LUT"s), either in hardware memory or in software, using the numerical image data from a display memory to access predetermined values in the R, G and B tables. In the case of a hardware-based LUT system, the numerical values from the display memory are used as addresses to a second memory device in which the lookup values have been previously stored. The data output from this second memory device then becomes the new, modified pixel value. The image is processed in a serial fashion generally starting at the top left of the image, performing the lookup function on each pixel successively, until the entire image has been modified. The resulting R, G and B data streams may be stored in another memory device, or connected to three digital to analog converters to produce three varying voltages to be connected to a component (RGB) color video monitor.
In some computer graphics systems, identical data is routed to all three LUTs. For example, consider an 8 bit display memory which is capable of representing 256 unique values. The 8 bit data stream would be routed to all three LUTs. By properly loading values into the tables in advance, the programmer can specify which colors, or mixtures of R, G, and B, each numerical value will produce. In such a system, only 768 bytes of memory are required. Because only 8 bits of pixel data are supplied, there are only 256 bytes in each table.
A straightforward application of the LUT technique to video special effects would involve decoding the composite video signal from a camera or video recorder into R, G, and B components. Three analog to digital converters would then be used to convert the signals into three parallel 8 bit streams of data, or 24 bits total. These 24 bits would then be routed to the address inputs of the lookup memory. The memory array would then produce three new streams of 8 bit R, G, and B data. Three digital to analog converters would convert these streams to three voltages representing the new R, G, and B values. A composite encoder would then return the image to the composite video form. Unfortunately, 24 bits of image data (8 bits each, R, G, and B) requires a LUT memory device of 48 Megabyte capacity. At this time, such a memory system is very expensive.
Digital video systems generally fall into two categories: component and composite. Component systems represent the image as three discreet data streams such as Red, Green, Blue (R, G, B) . Composite systems represent the image using a single data stream, usually 8 bits wide, which is formed by passing the NTSC composite video signal through a single analog-to-digital converter without any prior decoding. In the NTSC video signal, color is presented by a 3.58 MHz amplitude and phase modulated subcarrier which is mixed with the luminance, or black and white portion of the image. In a composite digital video system, this subcarrier becomes part of the data stream. Color information in a composite digital system, is represented indirectly by the relationships between successive pixels, rather than explicitly as a proportion of R, G, and B values in the component digital video system.
Since there is only one data stream in the composite digital system, and color information is not directly accessible, applying the LUT concept would be of limited usefulness. A system could be constructed similar to a computer graphics system mentioned above, in which the 8 bit data stream could be connected to individual R, G, and B LUT memories. The outputs of these tables could be routed to respective red, green, and blue digital to analog converters. The resulting component analog video signals could then be fed to an NTSC composite encoder for viewing on a standard TV monitor. This would allow certain false color effects as well as contrast and brightness manipulation. However, the color information present in the original image is lost in the process. All effects would effectively be taking place on the luminance, or monochrome image. This system also requires considerable circuitry to convert and encode the composite video.
In the NTSC color television system, 4 fields are necessary in order to produce the desired color. Fields 1 and 2, each comprised of 262.5 lines, are interlaced, to produce one frame. Fields 3 and 4 are usually, if there is no movement, identical with fields 1 and 2, respectively, with the exception that the color reference subcarrier phase is reversed with respect to fields 1 and 2. It is desired to devise an inexpensive and simple system for creating special color changes and effects in, and utilizing, a four field, or multi-field color system, or other similar system, such as the PAL system.
It would be advantageous to devise a color lookup table system for color for providing color effects which could be used with a composite digital video system and produce a single composite digital data stream. Such a system would require no error-prone color encoders and decoders and only one analog to digital converter and one digital to analog converter, and would be simpler and less expensive.
It is therefore an object of this invention to provide a LUT circuit for producing color effects which will operate within the composite digital video domain, avoiding the necessity of decoding or encoding circuits to convert the signal to a component format to provide a less expensive and simpler method and apparatus for producing color effects on a composite video signal and that allows for versatile application to obtain numerous and interesting color effects. It is further an object to provide such a system that requires less memory and less processing times; it is further desired to have such a system capable of operating within a system, having internally computer generated sync signals.
Other objects and features of the invention and the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings which exemplify the invention, it being understood that changes may be made in the specific method and apparatus disclosed herein without departing from the essentials of the invention set forth in the appended claims.